Thionyl chloride (SOCl.sub.2) is a valuable and useful chemical compound widely used in the preparation of chemical intermediates for pharmaceuticals, dyestuffs and other chemical areas. Purity of thionyl chloride is important as the impurities present create side reactions that complicate the chemical processes.
Thionyl chloride is prepared by the reaction of an oxy compound with sulfur chlorides or sulfur and chlorine. The impurities that are found in the manufacturing process are mainly mixtures of sulfur chlorides, e.g. sulfur monochloride and sulfur dichlorides, sulfuryl chloride, sulfur dioxide and chlorine.
Purification of thionyl chloride by distillation should be relatively easy due to the boiling point differences of the contents:
______________________________________ Thionyl chloride 78.degree. C. Sulfuryl chloride 69.degree. Sulfur monochloride 138.degree. Sulfur dichloride 59.degree. Sulfur dioxide -10.degree. Chlorine -35.degree. ______________________________________
The chlorine and sulfur dioxide are easily separated, but the problem of thionyl chloride purification is compounded by the fact that the presence of sulfur chlorides present in the manufactured product cannot be separated by distillation as the higher chlorides of sulfur dissociate at distilling temperatures to form the lower chlorides, which can be grossly removed but not completely. It has been found that complete purification by distillation is very tedious, requiring many steps, i.e. removing gross amounts, followed by operation at total reflux until all the residual sulfur chlorides are decomposed to sulfur and chlorine. Decomposition of the thionyl chloride itself may occur during such a prolonged process, hampering purification and reducing the yield.
Methods of purification of impure or crude thionyl chloride have been developed wherein organic materials that will react with the sulfur chlorides, are added to the impure thionyl chloride and then separated. These methods have been unsuccessful in producing high purity material as the additives generally remain as impurities in the thionyl chloride and generally increase the organic carbon content of the product.
The addition of sulfur to the impure thionyl chloride during separation has been disclosed in Kunkel, U.S. Pat. No. 3,155,457 wherein sulfur is added to the impure material in the distillation pot as well as to the vapor stream near the top of the distillation column. This process aids in the conversion of the sulfur chlorides to the sulfur monochlorides which has the highest boiling point of the sulfur chlorides and will return to the distillation pot. This process does not remove all of the offending sulfur chlorides but does reduce their level in the collected thionyl chloride.
A method of purification disclosed in Bohm, U.S. Pat. No. 3,592,593 essentially utilizes the above purification process but adds an active iron material to the sulfur that is added to the distilling column to increase the conversion of the sulfur dichloride to sulfur monochloride.
Although both of the above processes reduce the content of the impurities in the crude thionyl chloride, neither produce very high purity thionyl chloride, as neither process specifically affects the sulfuryl chloride content. The invention disclosed herein removes the sulfuryl chloride from the distillate producing a high purity thionyl chloride with a thionyl chloride content of 99.7+%.
An object of this invention is to provide a method of purification of thionyl chloride that is continuous and efficient.
Another object of this invention is to provide a thionyl chloride purification system that will produce a water-white thionyl chloride with only trace amounts of sulfur chlorides remaining.